The present invention relates generally to material testing and, more particularly, to a device and method for non-contact detection of structural and/or surface faults in large-surface bodies.
In the production of large-surface homogeneous materials, or those composed of various components, inclusion of air, cracks or inhomogeneities in he material itself or between the individual components may occur. These structural weaknesses generally reduce the adhesive power and therefore the quality of the product. It is therefore desirable to recognise these faults early on and if possible during the course of the manufacturing process, so as to be able to intervene in the manufacturing process to correct them.
Depending on the type and size of the material to be examined at the moment various procedures are used to detect weaknesses in material:
Systems which measure in points, such as, for example, ultrasound, radiometric, inductive or capacitive methods, give information on the weaknesses in a material in a small measuring field. To enable a 100% check of large-surface material with these methods, procedures measuring in points have to be conducted over the entire object, which is time-consuming. These procedures which measure in points are not usually suitable for use for a 100% on line manufacturing check because of the time-consuming scanning of the specimen. Simultaneous parallel operation of several appliances admittedly usually shortens measuring time, but makes the measuring devices correspondingly expensive.
Fast systems which measure surfaces extensively are desirable for 100% manufacturing control. Optical inspection of the surface in visible light allows, e.g. faults in the surface layer to be detected. Interferometric measuring systems, such as, e.g. the Electronic Speckle Model Interferometer (ESPI) or the Shear ESPI can measure the whole area for faults even inside material specimens. However, owing to their great sensitivity to outside disturbing influences they cannot be easily and reliably integrated into the production process. Microwaves or radio waves would in principle be suitable for a monitoring process of this kind, but require relatively high expenditure to comply with the strict safety regulations for use with sources of radiation.
DE 197 03 484 A1 discloses a test procedure in which internal faults are detected in a material or material composite, by the generation of a heat flow in the area to be examined, which is disrupted at defective spots. The resulting distortions of the surface temperature field are detected and evaluated for fault contrasting. The sensitivity to smaller, deeper lying internal faults is achieved by optimized heat conduction and by use of a high-resolution measuring device to detect the local and temporal surface temperature distribution.
DE 196 28 391 C1 discloses a signal-processing unit of a device for photothermal testing of a surface of a test sample. This provides a speed measuring device with which the relative speed of a test sample can be defined with regard to the optical part as well as a detector firmly attached thereto. With the thus gained measured speed value the temporal course of induced heat radiation generated by excitation radiation on a static test area can be corrected.
DE 197 20 461 A1 discloses a procedure and a device for monitoring the internal cooling structure of a turbine blade. An initial thermal image of the turbine blade to be tested is recorded by means of a first infrared camera. The turbine blade is then heated up for a short time by blowing hot air into its cooling structure. A thermal image of the thus heated up turbine blade is then recorded by a second infrared camera. The infrared thermal imaging camera can work in a line-scan mode, in other words scan the object by line. The temporal course of the temperature distribution recorded by the camera is digitalized on line by a personal computer by means of an appropriate evaluation and image-processing program and the initial thermal image subtracted therefrom in each case. These differential images are compared and assessed with pictures of a reference blade.
H. Tretout""s treatise: xe2x80x9cComposites, la thermographie ca marche en CNDxe2x80x9d MESURES, REGULATION AUTOMATISME, Vol. 51, No. 15, November 1986 (1986-11), pages 43-46, XP002125600 Paris, discloses a device in which the test sample is conveyed with the aid of a conveying device, not described in more detail, within a perpendicular plane past a horizontally radiating heat source and a thermal imaging camera positioned behind it, which has only one camera line.
The present invention is directed to overcoming one or more of the problems set forth above.
An aspect of the invention is to provide an improved procedure for non-contact detection of structural and/or surface faults in large surface bodies.
In accordance with the above aspect of the invention, there is provided a device that includes a conveying device with an approximately horizontal conveying plane and an unsupported area with respect to the test sample to be conveyed; a heat source arranged above the conveying plane over the unsupported area; a thermal imaging camera, which has several camera lines arranged in each case transversely to the direction of conveyance and in succession in the direction of conveyance, arranged above the conveying plane; and, a computer with monitor capable of using information recorded in succession from the camera lines to construct heat images coming from various depth planes of the test sample.
With the thermographic inspection appliance both defects on the surface of large board-shaped materials and also faults near the surface to a depth of several centimeters can be detected at great speed without contact and without destruction. This allows, for example, a thermographic picture of a coated chipboard to be produced, the surface of which has been homogeneously heated by a few degrees. After a short heat penetration time from each individual line of the thermal imaging surface camera a complex heat image model, rich in contrast, composed from various depth planes of the test sample, can be observed on the surface of the test sample. From these images conclusions can be drawn as to the quality of the gluing and the poor adhesive power of the material associated with it.